134 



METHODS OF ANALYSIS. 



(3) DETERMINATION. 



Prepare the disks of fat as follows : Allow the melted and filtered fat to fall 

 from a dropping tube from a height of from 15 to 20 cm onto a smooth piece 

 of ice in boiled distilled water. The disks thus formed are from 1 to 1.5 cm 

 in diameter and weigh about 200 mg. By pressing the ice under the water 

 the disks are made to float on the surface, whence they are easily removed 

 with a steel spatula, which should be cooled in the ice water before using. 



The disks must be allowed to stand for two or 

 three hours in order to obtain the normal melting 

 point. 



Place the test tube containing the alcohol and 

 water in a tall beaker containing ice and water 

 and leave it there until cold. Then drop the 

 disk of fat into the tube from the spatula, and 

 it will at once sink until it reaches a position 

 where the density of the alcohol-water is exactly 

 equivalent to its own. Lower a delicate ther- 

 mometer into the test tube until the bulb is 

 just above the disk. In order to secure an even 

 temperature in all parts of the alcohol mixture 

 in the vicinity of the disk, move the thermometer 

 from time to time in a circle. The disk having 

 been placed in position, slowly heat the water in 

 the beaker, stirring it constantly by means of 

 the blowing apparatus already described. 



When the temperature of the alcohol-water 

 mixture rises to about 6 C below the melting 

 point the disk of fat begins to shrivel and 

 gradually rolls up into an irregular mass. 

 Lower the thermometer until the fat particle 

 is even with the center of the bulb, which 

 should be small so. as to indicate only the tem- 

 perature of the mixture near the fat. Give a 

 gentle rotary motion to the ther- 

 mometer bulb, and so regulate 

 the temperature that the last 2 

 C of the increment require about 

 ten minutes. The mass of the 

 fat gradually assumes the form 

 of a sphere, and when it has 

 gathered in a ball make the read- 

 As soon as the temperature is taken remove the tube 

 At once place in the bath a 



FIG. 6. Apparatus for the determination of melting 

 point. 



ing of the thermometer. 



from the bath and place it again in the cooler, 

 second tube containing alcohol and water. The test tube is of sufficiently low 

 temperature to cool the bath to the desired point, ice water having been used as 

 a cooler. After the first determination, which should only be a trial, the tem- 

 perature of the bath should be so regulated as to reach a maximum of about 

 1.5 above the melting point of the fat under examination. 



Do not allow the edge of the disk to touch the sides of the tube. This acci- 

 dent rarely happens, but in case it should take place and the disk adhere to 



